Microfiltration and ultrafiltration are two recognized types of membrane separation processes. See Membranes: Learning a Lesson from Nature, Koros, W. J., Chemical Engineering Progress, October 1995, pp. 68-80, the disclosure of which is incorporated herein by reference. These processes are known for such representative utilities as processing corn-stillage streams, concentrating emulsions and cell suspensions, reducing bacteria and particulate turbidity, recovering paint, removing oil microemulsion and separating biomolecules and virus from aqueous streams.
In microfiltration and ultrafiltration the mechanism for separation involves sieving of primarily liquid feed streams containing suspended species through a microporous membrane. The driving force for separation is a transmembrane pressure differential, i.e., the feed stream side is placed at a higher pressure than the filtrate stream side to force the liquid through the membrane pores. The transmembrane pressure gradient can be created by applying a pressure to the feed and/or by drawing a vacuum on the filtrate. Of course, suspended species of size larger than the membrane pores are rejected which yields a filtrate free of large species and a retentate stream concentrated in the rejected species.
Microfiltration and ultrafiltration suffer from the serious drawback that the membrane tends to foul over time in service. That is, as filtration continues the pores become blocked which reduces and ultimately stops the separation process until the foulant is cleaned if possible, or the fouled membrane is replaced with virgin membrane.
Fouling of microporous membranes in microfiltration and ultrafiltration has been studied extensively. While the mechanisms and theories concerning fouling are very complex, two general categories have been identified, namely deposition and adsorption fouling phenomena. Deposition fouling occurs as a result of hydrodynamic forces. The pressure gradient across the membrane actively pushes the foulant species into the pores. Adsorption fouling relates to the adhesiveness between the foulant and the membrane. Generally, suspended species to be separated from the feed liquid that have great affinity for the membrane material tend to adhere to the membrane at the surface and in the pores. The bulk of foulant species settling on and in the membrane prevents further transmembrane flow of liquid.
Adsorption fouling, and to some extent deposition fouling, can be affected by the chemistry of the feed stream and membrane system. For example, electrically charging the membrane can sometimes effectively mitigate fouling by polar species from nonpolar liquid or vice versa. In many practical separations, however, the species to be separated possess a wide range of polarity. Therefore, membrane charging is useful in those specific separations to which it is amenable.
It is highly desirable to have a microfiltration and/or ultrafiltration process that is resistant to fouling in a wide variety of feed stream composition systems. Hence, according to the present invention there is now provided a method of filtering a suspension comprising the steps of
contacting one side of a two-sided microporous membrane structure with a suspension in an aqueous medium, the structure having passageways through the structure of a size effective to reject species suspended in the aqueous medium of size in the range of about 0.01-10 .mu.m; PA1 creating a transmembrane pressure gradient effective to cause aqueous medium to pass through the microporous membrane structure to the other side of the membrane structure to form a filtrate substantially free of rejected species; and PA1 removing the filtrate, in which at least a portion of the microporous membrane structure in contact with the suspension comprises a substance having a surface energy less than that of tetrafluoroethylene homopolymer.